Vinylidene fluoride based polymers, collectively VDF-copolymers, are unique offering the widest possible range of processing options to obtain articles having the beneficial attributes associated with improved use temperature, chemical resistance associated with the high concentration of carbon fluorine bonds. Homopolymers of VDF offer high melting temperature of over 160° C., and even closer to 170° C. It is generally known that melting temperature diminishes as higher comonomer content is incorporated in to the PVDF backbone. This is expected in semi-crystalline polymers such as PVDF—since crystallinity generally decreases (along with melting temperatures) with increasing comonomer content. For example, U.S. Pat. No. 6,586,547 discloses that copolymers having more than about 30 wt % comonomer have zero crystallinity or no melting temperature.
Copolymers of VDF and hexafluoropropylene (HFP) show the expected relationship between comonomer content, crystallinity and inciting temperature. U.S. Pat. No. 3,051,677 describes batch emulsion and continuous emulsion processes for the copolymerization of vinylidene fluoride and 30 to 70 weight percent of hexafluoropropylene monomer. Under both sets of reaction conditions, relatively low crystallinity and low melting temperature is confirmed by the exemplified properties.
U.S. Pat. No. 3,178,399 describes both batch and continuous emulsion processes for preparing HFP-VDF copolymers having between 85 and 99 mole percent VDF and 1 and 15 mole percent HFP. Once again, the synthesis techniques described inevitably produce copolymers having relatively low melting temperature even at high crystallinity.
Polymers and copolymers of 2,3,3,3-tetrafluoropropene copolymerized with vinylidene fluoride are known in the art. The polymerizations can be carried out in a batch mode (U.S. Pat. No. 2,970,988, U.S. Pat. No. 3,085,996). They polymerization can also be carried out in a semi-batch mode (U.S. Pat. No. 6,818,258, U.S. Pat. No. 7,803,890, US 2008153977)—each of which disclose the ratio of the comonomers in the initial charge to be the same as the ratio of the comonomers in the delay feed. In WO10005757 the semi-batch polymerization process is disclosed having either the same initial charge and delay feed monomer ratios, or else a reaction charged initially only with VDF—leading to an initial production of PVDF homopolymer.
U.S. Pat. No. 2,970,988 discloses that the ratio of 2,3,3,3-tetrafluoropropene to VDF in the over-all polymer (initial charge in a batch reaction) determines the physical properties of the copolymer macrostructure.
The polymer chain microstructure of PVDF-based polymers relates to the crystalline and amorphous regions of the polymer. The relationship between the amorphous and crystalline regions, as well as the amount of crystalline phase, affect the mechanical properties of the copolymer and determine the ultimate applications for a given resin composition. At one end of the spectrum there are totally amorphous thermoplastic polymers and at the other extreme the highly crystalline polymers. The microstructure of the polymer chain determines the melting temperature at a given crystalline content. This attribute is controlled by the type and amount of the crystalline phase and copolymer microstructure.
It has now been found that while the overall ratio of the 2,3,3,3-tetrafluoropropene and VDF comonomers in the copolymer (macrostructure) is an important consideration, the microstructure of the copolymer can be adjusted to create a wide range of properties within the same overall comonomer ratio. This unexpectedly allows one to create copolymers that have a very high melting point with a low crystallinity, or a low melting point with high crystallinity from the same macrostructure monomer ratio.
One of the more interesting, and unexpected, controlled microstructure 2,3,3,3-tetrafluoropropene/VDF copolymers of the invention is that having a high melting temperature and high flexibility (low crystallinity), and the ultimate end uses of this group of copolymers. Since higher melting temperature is usually associated with higher crsytallinity in semi-crystalline polymers, the creation of a high melting temperature polymer with low crystallinity is unexpected. The high melting point elastomer of the invention offers unique opportunities for end use applications. 2,3,3,3-Tetrafluoropropene/VDF copolymers with these properties are not known in the art, and provide fluorinated thermoplastic having a unique combination of properties including excellent flexibility, high melting temperature, high clarity, and solution stability.